Re: [LTP] LTP RT Tests (Cyclic, rt-migrate, etc)

[Gregory Haskins <ghaskins@novell.com>]

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Steven Rostedt wrote:
> On Fri, 10 Jul 2009, Darren Hart wrote:
>
>   
>> Subrata Modak wrote:
>>     
>>> On Fri, 2009-07-10 at 22:00 +0530, Sripathi Kodi wrote:
>>>       
>>>> On Wednesday 08 July 2009 23:43:53 Subrata Modak wrote:
>>>>         
>>>>> Darren/Sri/Gowri,
>>>>>
>>>>> Where do you want me to put this exactly inside the RT tree ?
>>>>>           
>>>> Hi Subrata,
>>>>
>>>> Going by how the tests are organized currently, I think this should go
>>>> into it's own directory under testcases/realtime/func. We will need to
>>>> add a makefile to it. Are you looking at us to help you with this?
>>>>         
>>> Correct. Please send me a patch which integrates it into RT tests build,
>>> install & run.
>>>       
>> Just got back from a week vacation and am burning through mail as fast as I
>> can :-) Haven't had a look yet, but does this test use librttest.h?  I suspect
>> not.  We'll need to adapt it to run within the existing ltp real-time testing
>> framework, which includes things like buffered output as well as mlocking
>> support.
>>
>> Lastly, I'm not sure this test does anything effectively different than
>> prio-wake, already in the tree.

Just to add to Steven's comments below:  At the time that rt-migrate was
written, LTP and others lacked sufficient resolution in their testing to
reliably find the type of problem that rt-migratate can pinpoint
quickly.  IIRC, "football" was potentially capable of finding these
types of scheduler bugs, but it often failed to find it at all, or it
took 24h+ of runtime to find it.  Steven's test could find it in under a
second or two.  And, as Steven mentions below, rt-migrate is
additionally designed to look at the top N prio tasks (where N = cpu-count)

That said, I am not familiar with "prio-wake" so I am not sure if its
new or if it has direct overlap with Steven's test or not.

>>   My other concerns with the test are its
>> explicit 1ms preemption criteria (as Steven described it anyway).  We are
>> trying to move away from criteria being inherent in measurement tests, and 1
>> ms seems like an awfully long priority inversion to be an acceptable criteria
>> to many users.
>>
>> Steven, am I missing something conceptually here?
>>     
>
> Hmm, I missed this email, sorry for the late reply.
>
> What does prio-wake do?
>
> This test is what I used to develop the rt scheduler in mainline (as well 
> as in -rt).  It wakes up N+1 tasks with lowering real time priorities. 
> Where N is the number of CPUs in the system. Then it makes sure that the 
> these tasks spread out across the CPUs. Most tests just test the highest 
> priority task in the system. But those tests usually miss the second 
> highest prio task in the system. If you have a second highest prio task in 
> the system and a CPU is available to run, then it should run on that CPU. 
> But what happens is that it can wait to be migrated and can take millisecs 
> to wake up.
>
> This test makes sure that all the high prio tasks that are in the running 
> state are actually running on a CPU if it can.
>
> Make sense?
>
> (BTW, current -rt and mainline now fail this test :-? )
>
> -- Steve
>
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